I'm cross dominant but find I have no trouble hitting the center.
Most RH shooters that hit left have too much finger in the trigger guard, and/or they are squeezing their entire hand as the gun fires. With Sigs they have a long trigger pull and a long reset, the DA pull exacerbates it.
You want to only use the tip of your finger. Basically the part of the finger with the fingernail. If you are at the joint, you will pull the shot on anything that does not have a very short trigger pull.
Hand positioning on the gun is what worked the best for me, and this has alot to do with how my trigger finger interacts with the trigger. You want the gun in the middle of your hand. If you hold the gun straight down you should have a straight line between the muzzle and your forearm. This also has the benefit of reducing recoil and stress on your wrist.
Your trigger pull should be straight back every time. If you make the gun go off at the last second you will tend to pull the shot. Generally you can even get a grouping out of this, but will tend to push a shot high, or dump one every so often.
Breathing is bull**** in my opinion. You will be hyperventilating or holding your breath when you are in that situation. I held my breath, and it was purely involuntary. You will result 100% to what you train yourself to do. Breathing is used by hunters because they are in ambush and have the luxury of time. When someone is 9' away from you and that sight comes on target, the only thing in your mind should be sight-picture, pull straight, sight-picture, pull straight, until that target goes down and out, whilst you are moving to cover.
I agree 100% with the person that said practice at 3 yards (10'). Practice acquiring your sights and focus more on your mechanics than the shot placement. In fact, try to shoot an entire magazine without moving your focal point from the front sight.